Atomic layer deposition (ALD) is an enabling technology for next generation conductor barrier layers, high-k gate dielectric layers, high-k capacitance layers, capping layers, and metallic gate electrodes in silicon wafer processes. ALD has also been applied in other electronics industries, such as flat panel display, compound semiconductor, magnetic and optical storage, solar cell, nanotechnology and nano materials. ALD is used to build ultra thin and highly conformal layers of metal, oxide, nitride, and others up to one monolayer at a time in a cyclic deposition process. Oxides and nitrides of many main group metal elements and transition metal elements, such as aluminum, barium, cerium, dysprosium, hafnium, lanthanum, niobium, silicon, strontium, tantalum, titanium, tungsten, yttrium, zinc and zirconium have been produced by ALD processes using oxidation or nitridation reactions. Pure metallic layers, such as Ru, Cu, Ti, Ta, W, and others may also be deposited using ALD processes through reduction or combustion reactions.
A typical ALD process is based on sequential applications of at least two precursors to the substrate surface with each pulse of precursor separated by a purge. Each application of a precursor is intended to result in up to a single monolayer of material being deposited on the surface. These monolayers are formed because of the self-terminating surface reactions between the precursors and surface. In other words, reaction between the precursor and the surface should proceed until no further surface sites are available for reaction. Excess precursor will not react with same type of precursor that has already reacted at the surface site and the excess precursor is then purged from the deposition chamber. Following the purge, the second precursor is introduced and reacts with the first precursor on the surface. Any excess second precursor is purged after surface saturation. Each precursor pulse and purge sequence comprises one ALD half-cycle that theoretically results in up to a single additional monolayer of material. Because of the self-terminating nature of the process, even if more precursor molecules arrive at the surface, no further reactions will occur. It is this self-terminating characteristic that provides for high uniformity, conformality and precise thickness control when using ALD processes.
Chemical vapor deposition (CVD) is a chemical process used to produce high-purity, high performance solid materials, particularly in the semiconductor and microelectronic industries for the production of thin films with greater growth rates than ALD processes. A typical CVD process comprises exposing a wafer to one or more volatile precursor materials that react or decompose on the wafer surface to produce the desired layer deposit continuously. Volatile by-products are usually produced that must be removed by gas flow through the reaction chamber. CVD is used in a wide variety of forms, including metalorganic chemical vapor deposition (MOCVD) to deposit materials such as monocrystalline, polycrystalline, amorphous and epitaxial layers of silicon, carbon fiber, carbon nanofibers, filaments, carbon nanotubes, silicon dioxide, silicon germanium, tungsten, silicon carbide, silicon nitride, silicon oxynitride, titanium nitride, high-k dielectrics, etc. MOCVD techniques are based upon the use of metalorganic precursor materials for the formation of the desired layers.
There are a wide variety of precursor materials that can be used in ALD and MOCVD processes. For example, new solution based precursors have been proposed in Published US patent application 2006-0269667, hereby incorporated in its entirety by reference. These solution based precursors allow greater choice in precursor materials and because they are comprised of a dissolved metal precursor in a solvent mixture, can also enhance chemical utilization and reduce costs.
In particular, disclosed in the above published patent application are precursor solutions comprised of one or more low volatility precursors (including solid precursors) dissolved in a solvent. For example, the precursor may be a halide, alkoxide, β-diketonate, nitrate, alkylamide, amidinate, cyclopentadienyl, or other organic or inorganic metal or non-metal compound. More specifically, the precursor may be Hf[N(EtMe)]4, Hf(NO3)4, HfCl4, HfI4, [(t-Bu)Cp]2HfMe2, Hf(O2C5H11)4, Cp2HfCl2, Hf(OC4H9)4, Hf(OC2H5)4, Al(OC3H7)3, Pb(OC(CH3)3)2, Zr(OC(CH3)3)4, [(t-Bu)Cp]2ZrMe2, Ti(OCH(CH3)2)4, [(t-Bu)Cp]2TiMe2, [(i-Prop)Cp]3La, Ba(OC3H7)2, Sr(OC3H7)2, Ba(C5Me5)2, Sr(C5i-Pr3H2)2, Ti(C5Me5)(Me3), Ba(thd)2*triglyme, Sr(thd)2*triglyme, Ti(thd)3, RUCP2, Ta(NMe2)5 or Ta(NMe2)3(NC9H11). The concentration of the precursor in the precursor solution is generally from 0.01 M to 1 M and the precursor solution may include stabilizing additives with concentrations from 0.0001 M to 1 M, such as oxygen containing organic compounds, e.g. THF, 1,4-dioxane, or DMF. The solvent used has a boiling point selected to ensure no solvent loss during vaporization. In particular, the solvent may be dioxane, toluene, n-butyl acetate, octane, ethylcyclohexane, 2-methoxyethyl acetate, cyclohexanone, propylcyclohexane, 2-methoxyethyl ether(diglyme), butylcyclohexane or 2,5-dimethyloxytetrahydrofuran. Specifically disclosed precursor solutions include aluminum i-propoxide dissolved in ethylcyclohexane or octane; [(t-Bu)Cp]2HfMe2 dissolved in ethylcyclohexane or octane; Tetrakis(1-methoxy-2-methyl-2-propoxide)hafnium (IV) dissolved in ethylcyclohexane or octane; hafnium tert-butoxide or hafnium ethoxide dissolved in ethylcyclohexane or octane; a mixture of Ba(O-iPr)2, Sr(O-iPr)2, and Ti(O-iPr)4 dissolved in ethylcyclohexane or octane; and RuCp2 dissolved in dioxane, dioxane/octane or 2,5-dimethyloxytetrahydrofuran/octane.
Further solution based precursors are disclosed in pending U.S. patent application Ser. No. 12/261,169, hereby incorporated in its entirety by reference. This application discloses precursors comprising lanthanum alkyl cyclopentadienyl compounds. Specific examples of such compounds include lanthanum(III)isopropoxide; tris(N,N-bis(trimethylsilyl)amide)lanthanum; tris(cyclopentadienyl)lanthanum; or tris(isopropyl-cyclopentadienyl)lanthanum.
These solution based precursors are delivered by direct liquid injection (DLI) methods, which means the precursor liquid can remain at ambient temperature, thereby providing long stability and dosage control. The solution precursor is vaporized in a vaporizer before deliver to the process chamber. When a standard liquid precursor container is used as the container for the source material precursors, the liquid material is pushed into the DLI system through a dip-tube by head-space gas pressure. The liquid precursor in pure chemical form or the dissolved metal precursor in solution form might disadvantageously interact with downstream DLI metal surfaces, such as particle filters, liquid mass flow controllers, valves, and pipes to form thin film layers on the surfaces thereof. If such interaction occurs, particles could form from the adsorbed metal complex when the DLI system pressure and temperature change. In addition, air and moisture contamination during the source change or system service may promote unwanted surface reactions to form thin film or particles. This thin film and particle formation leads to problems with the DLI system, particularly, making it difficult to control liquid flow, and eventually causing system clogging.
The source condensed film or absorbed metal organic precursors cannot be purged out of the system with inert gases and vacuum cycles because of the surface bonding that occurs. Therefore, removal of surface molecules generally requires washing and dissolving in a cleaning solvent before purge with air or moisture. Such a system requires additional piping be included in the standard DLI system or disconnection of the precursor source container to enable cleaning.
Therefore, there remains a need in the art for improvements to the use of liquid based precursors in ALD and MOCVD processes.